Mobile terminals have evolved from the clumsy and cumbersome bag phones of the 1970s and 1980s into sleek, minute combined cellular phone/personal digital assistants that offer a host of services. As the mobile terminals have evolved, so have the communication protocols through which the mobile terminals communicate. The evolution of the communication protocols has seen different standards and different frequency bands used for the mobile terminals. Most of the older analog standards are being phased out and replaced with more robust digital and/or wideband standards such as wideband code division multiple access (WCDMA) or enhanced general packet radio service (EGPRS).
As a result of the various frequency bands and protocols available, many mobile terminals have been made which are designed to operate in multiple protocols and/or multiple frequency bands. These multimode or multiband mobile terminals have proven popular because they allow a user's mobile terminal to operate in more environments and consequently have better coverage than single mode/single band mobile terminals. Since the point of the mobile terminal is to enable the user to remain available, better coverage helps insure that the user is available in more locations. Initially, the multimode mobile terminals included separate antennas and separate receivers for each mode in which the mobile terminal was designed to operate. As is readily imagined, these separate receivers increased the size of the mobile terminal, and the extra components reduced battery life expectancy.
The next evolution of the mobile terminal was to combine the separate antennas into a single antenna and switch between receivers. While this arrangement proved adequate for early generation mobile terminals, all code division multiple access (CDMA) standards impose strict requirements on power levels emitted from the mobile terminal antenna. In addition the full duplex nature of CDMA and WCDMA require strict control on noise at the receive frequency leaking into the receive path requiring very rigorous filtering at the output of the power amplifier. A typical WCDMA filter at the output of the power amplifier attenuates adjacent receive channels by approximately 45 dB.
When the requirements for WCDMA are implemented into a multimode mobile terminal having a switch at the antenna that facilitates switching between modes by switching between transceiver elements, the switch must attenuate noise signals from the non-WCDMA mode transceiver elements by 45 dB so that noise from the non-WCDMA elements does not interfere with the WCDMA components. Designing a switch that imposes this level of attenuation is a non-trivial exercise. The typical solution is a series of switches. The series of switches has several disadvantages. First, each switching element has an associated loss. Thus, the series of switches has a relatively high loss characteristic. To compensate for the losses in the switches, the power amplifier of the mobile terminal has to boost the transmitted signal more than would be necessary in the absence of the series of switches. Second, the increase in output levels from the power amplifier requires more current drain from the battery, which accelerates drainage of the battery. Therefore, indirectly, high switch loss accelerates drainage of the battery. Third, the additional power amplifier output power required to overcome the switch insertion loss and energy dissipated in the switches creates heat. The heat generation may make the mobile terminal too hot for a typical user to handle comfortably.
One relatively recent improvement in mobile terminal power amplifiers is the use of a balanced power amplifier. Sometimes referred to as a quadrature amplifier, a balanced amplifier includes a splitter, which typically shifts one of the split signals by ninety degrees. Each of the split signals is then amplified by an amplifier chain, and the amplified signals are combined by a combiner. While the combiner has two output ports, only one output is generally used. The other output port, usually called the isolated port, is grounded. While balanced power amplifiers are useful tools, balanced power amplifiers are currently under-utilized by the mobile terminal industry.
Thus, there is a need for an improved technique for isolating different modes of a multimode mobile terminal from one another so that the presence of a first mode's operational components does not significantly degrade a second mode's performance and vice versa.